This invention is in the field of networks for carrying optical signals. The invention relates to apparatus for multiplexing or demultiplexing a number of optical bands. The invention relates more particularly to such apparatus which incorporates arrayed waveguide gratings.
Optical fibers provide a way to transmit large volumes of data from place to place. It is often desirable to wavelength multiplex a number of signals onto a single optical fiber. This can be accomplished by passing each signal into a different input of a multiplexer and connecting an output of the multiplexer to the optical fiber. The signals can be recovered at a destination by demultiplexing.
Arrayed waveguide grating routers (AWGRs) are commonly used as multiplexer/demultiplexers in such systems (the same device can typically be used either as a multiplexer, as a demultiplexer, or simultaneously as a multiplexer and a demultiplexer). An arrayed waveguide router uses an arrayed waveguide grating to separate signals by wavelength. Example AWGRs are described in U.S. Pat. Nos. 5,002,350 and 5,136,671, both invented by Dragone.
FIG. 1 shows the main functional parts of a simple prior art AWGR 10. AWGR 10 comprises a pair of free propagation regions which are typically implemented as slab waveguides 20 and 30. The free propagation regions are sometimes referred to as xe2x80x9cstar couplersxe2x80x9d. A number, M, of input waveguides 22 couple corresponding input ports 26 to input slab waveguide 20. A number, N, of output waveguides 32 couple slab waveguide 30 to a number of corresponding output ports 36. In the example illustrated in FIG. 1, M=5 and N=5. Input waveguides 22 and ports 26 are labeled with the index p with 0xe2x89xa6pxe2x89xa64. Output waveguides 32 and ports 36 are labeled with the index q with 0xe2x89xa6qxe2x89xa64. The terms xe2x80x9cinputxe2x80x9d and xe2x80x9coutputxe2x80x9d are used herein for reference only. Light can propagate in either direction through AWGR 10.
Waveguides 20 and 30 are coupled to one another by a plurality of grating waveguides 16. Typically grating waveguides 16 each have a different length. The lengths of grating waveguides 16 are spaced from one another by predetermined amounts. Typically light enters AWGR 10 through at least one of input waveguides 22 and, in sequence, propagates through input slab waveguide 20, grating waveguides 16, output slab waveguide 30, and at least one output waveguide 32.
In the AWGR 10 of FIG. 1, light having a wavelength xcex which enters AWGR 10 at a certain one of input waveguides 22 is preferentially coupled into a specific one of output waveguides 32. AWGR 10 has an optical passband associated with each pair of an input port 26 and an output port 36. In general, a port is a location at which a waveguide of AWGR 10 couples to an optical pathway that is external to AWGR 10. For example, a port might be a location at which an input or output waveguide couples to an optical fiber external to the PLC on which the AWGR is fabricated. Within each optical passband the optical transmission between input port p and output port q is optimized for a range of wavelengths that are centered at the nominal wavelength for the passband, xcex. This can be achieved by designing the AWGR to satisfy the following equation:
mxcex=nsxc3x97dIxc3x97sin(xcex8p)+nsxc3x97doxc3x97sin(xcex1q)+ncxcex94Lixe2x80x83xe2x80x83(1)
where:
xcex is the wavelength;
ns is the effective index of refraction of slab waveguides 20 and 30;
nc is the effective index of refraction of channel waveguides 16;
dI and do are the center-to-center separations of grating waveguides 16 at the points where they couple to input slab waveguide 20 and output slab waveguide 30 respectively;
xcex94Li is the difference in length between adjacent grating waveguides 16;
m is the diffraction order for a particular passband associated with an input port p and an output port q;
xcex8p is the angle between the point at which the pth input waveguide 22 couples to slab waveguide 20 and an axis, A, of the focal curve on which arrayed waveguides 16 couple to slab waveguide 20 as shown in FIG. 1; and,
xcex1q is the angle between the point at which the qth output waveguide 32 couples to slab waveguide 30 and axis A of the focal curve on which arrayed waveguides 16 couple to slab waveguide 30 as shown in FIG. 1.
For simplicity, in the following discussion it is assumed that dI=do=d. In general, dI and do can be different. Also for simplicity, the length difference xcex94Li between adjacent waveguides 16 of the arrayed waveguide grating is assumed to have a constant value xcex94L. By applying small angle approximations to the sine functions of Equation (1) the relationship of Equation (1) can be recast as:                                           θ            p                    +                      α            q                          =                              (                                                            m                  xe2x80x2                                                  n                  s                                            xc3x97              d                        )                    ⁢                      (                          λ              -                              λ                c                                      )                                              (        2        )            
where:
xcexc is the wavelength of light diffraction order m that will propagate from the center (or xe2x80x9cpolexe2x80x9d) of the input focal curve to the center (or xe2x80x9cpolexe2x80x9d) of the output focal curve (i.e. from xcex8p=0 to xcex1q=0); and,
mxe2x80x2 is given by:                               m          xe2x80x2                =                  m          ⁡                      (                          1              +                                                (                                                            λ                      c                                                              n                      c0                                                        )                                ⁢                                  (                                                            ⅆ                                              n                        c                                                                                    ⅆ                                              xe2x80x83                                            ⁢                      λ                                                        )                                                      )                                              (        3        )            
where:
nc0 is the value of nc for light of wavelength xcexc.
Equation (3) in volves the value   (            ⅆ              n        c                    ⅆ      λ        )
which is a function of xcex. In general, however,   (            ⅆ              n        c                    ⅆ      λ        )
varies slowly with wavelength and so, for most wavelengths of interest, the value of   (            ⅆ              n        c                    ⅆ      λ        )
can be approximated by its value for xcex=xcexc.
The basic construction of FIG. 1 can be customized for specific applications by altering: the locations at which input waveguides 22 and AWG waveguides 16 couple to input slab waveguide 20; the locations at which output waveguides 32 and AWG waveguides 16 couple to output slab waveguide 30; the dimensions of slab waveguides 20 and 30; and, the relative lengths of AWG waveguides 18. For example, it is known to provide a 1xc3x97N demultiplexer by providing an input waveguide located so that xcex8p=0 and output waveguides located at angular positions which satisfy the relationship:                               α          q                =                              (                                                            m                  xe2x80x2                                                  n                  s                                            ⁢              d                        )                    ⁢                      (                                          λ                0                            -                              λ                c                            +                              q                ⁢                                  xe2x80x83                                ⁢                Δ                ⁢                                  xe2x80x83                                ⁢                λ                                      )                                              (        4        )            
where:
q=0, 1, 2, 3, . . . Nxe2x88x921;
xcex94xcex is a constant; and,
xcex0 is the nominal wavelength of the passband for which p=0 and q=0.
In such implementations, the passbands lie on a wavelength grid. That is, the wavelengths of the passbands are centered at wavelengths given by:
xcex=xcex0+qxcex94xcexxe2x80x83xe2x80x83(5)
FIG. 2A is a block diagram of a 1xc3x976 demultiplexer. In FIG. 2A and the other block diagrams referred to herein the xe2x80x9cinputxe2x80x9d waveguides are the lines entering the block from the left, the xe2x80x9coutputxe2x80x9d waveguides are the lines leaving the block on the right, the sequence of output waveguides is the same as would be present in a physical device, and the sequence of input waveguides is reversed from that of the physical device (in the example of FIG. 2A there is no sequence of input waveguides because there is only one input waveguide). Each of the arrows within the block indicate light of a particular wavelength xcex being coupled from one of the input waveguides to one of the output waveguides. The slopes of the arrows within the block are proportional to the value of xcexxe2x88x92xcex0. So, for example, an arrow representing optical coupling of a signal with wavelength xcex0 extends, straight across the box.
A problem encountered in manufacturing such demultiplexers is that variations in manufacturing processes may cause xcex0 to depart, from its intended value. This results in reduced yields because manufactured demultiplexers having values for xcex0 falling outside of an acceptable range cannot be used.
Some prior art demultiplexers address this problem by providing several inputs, each corresponding to a different value of xcex0. An input which provides a value of xcex0 lying in the acceptable range can be selected. An example of such a construction provides three input waveguides which couple to slab waveguide 20 at angles given by:                               θ          g                =                              (                                                            m                  xe2x80x2                                                  n                  s                                            ⁢              d                        )                    ⁢                      (                          1              -              δ                        )                    ⁢          g                                    (        6        )            
where:
g is an integer in the range of xe2x88x921xe2x89xa6gxe2x89xa61; and, xcex4 is a value smaller than 1.
Output waveguides couple to output slab waveguides at angular positions given by:                               α                      q            ,            g                          =                              (                                                            m                  xe2x80x2                                                  n                  s                                            ⁢              d                        )                    ⁢                      (                                          λ                0                            -                              λ                c                            +                                                (                                      q                    +                    g                                    )                                ⁢                Δ                ⁢                                  xe2x80x83                                ⁢                λ                                      )                                              (        7        )            
where q=0, 1, 2, 3, . . . N+1. When a particular one of the input waveguides is used (i.e. a specific value is selected for g) then N of the output waveguides can be selected such that:
nsd(xcex8g+xcex1q,g)=mxe2x80x2xcex94xcexq+mxe2x80x2(xcex0xe2x88x92xcexc+xcex94xcexxcex4g)xe2x80x83xe2x80x83(8)
For each value of q, the passbands lie on a wavelength grid according to:
xcex=(xcex0+xcex94xcexxcex4g)+qxcex94xcexxe2x80x83xe2x80x83(9)
It can be seen that the wavelengths of all of the passbands can be shifted in increments of the small amount xcex94xcexxcex4g by selecting an appropriate one of the input waveguides. This is valuable because it allows drifts in xcex0 which result from manufacturing process variations to be compensated for. If the fabrication process yields an AWG with the intended value of xcex0 then the output waveguide set associated with g=0 can be used with the resulting passbands having wavelengths of the intended values given by Equation 5. Where variations in the fabrication process result in an AWG that has a value of xcex0 that differs from the intended value, the difference between the actual and intended values of xcex0 can be reduced by an amount equal in magnitude to xcex94xcexxcex4 by selecting either the output waveguide set associated with g=1 or with g=xe2x88x921. Reducing the difference by this amount may be sufficient to make an AWG meet specifications applicable for use in a particular application when it would otherwise not meet the specifications.
It is known in the prior art that a number of AWGRs may be combined in a single module to permit N input signals to be demultiplexed into Q output signals. Such Qxc3x97N AWGR modules are, however, undesirably complicated to manufacture and to manage.
U.S. Pat. No. 6,181,849 discloses a single AWGR configured to separate signals from two input waveguides into two output bands. As shown in FIG. 2B, the wavelengths within each band are interlaced with the wavelengths in other bands. Further, the disclosed scheme cannot readily be extended to more than two inputs.
There is a need for Qxc3x97N AWGRs which are more flexible than those provided in the prior art. There is a need for AWGRs which permit N input signals to be demultiplexed into Q output passbands.
This invention provides optical devices which make possible alternative ways to combine or separate optical signals. The use of devices according to the invention provides convenient alternatives to presently known optical circuits.
One aspect of the invention provides an optical apparatus comprising: a plurality of input waveguides; a first free propagation region optically coupled to the plurality of input waveguides; a second free propagation region; an arrayed waveguide grating optically coupling the first and second free propagation regions; and first, second and third output waveguides optically coupled to the second free propagation region. The output waveguides are coupled to the second free propagation region at angles xcex1axe2x88x921, xcex1a and, xcex1a+1 respectively. The first output waveguide is adjacent to the second output waveguide and the second output waveguide is adjacent to the third output waveguide. According to this aspect of the invention, R given by:   R  =      2    xc3x97          "LeftBracketingBar"                        (                                    α                              a                -                1                                      -                          2              ⁢                              xe2x80x83                            ⁢                              α                a                                      +                          α                              a                +                1                                              )                          (                                    α                              a                -                1                                      -                          α                              a                +                1                                              )                    "RightBracketingBar"      
has a value of 0.1 or more.
Another aspect of this invention provides an arrayed waveguide grating device which comprises: a first free propagation region; a plurality of input waveguides optically coupled to the first free propagation region; a second free propagation region; an optical grating comprising a plurality of unequal length grating waveguides optically coupling the first and second free propagation regions; and, N groups of Q sequential output waveguides coupled to the second free propagation region at angular locations xcex1q. q is an index which ranges over the values 0,1, . . . , Qxc3x97Nxe2x88x921. xcex1q changes monotonically as q increases. The arrayed waveguide grating device is characterized in that each of the N groups of output waveguides is associated with at least one of the input waveguides and for each of the N groups, angular spacings between adjacent output waveguides both belonging to the group are significantly less than angular spacings between any waveguide in the group and any waveguide in any other group. For each of the waveguides in each of the N groups and the associated input waveguide there exists a passband and a range of the wavelengths associated with the passbands for each of the N groups is non-overlapping with the range of passband wavelengths for other ones of the N groups.
In some preferred embodiments of the invention the passband wavelengths of the output waveguides are substantially on a frequency grid. In other preferred embodiments of the invention the passband wavelengths of the output waveguides are substantially on a wavelength grid.
A device according to the invention may be used in combination with an optical device operating as a 1xc3x97N demultiplexer having N outputs. Each of the outputs is optically coupled to one of the input waveguides associated with a different one of the N groups. An optical switch may be provided in each of one or more of the optical paths which extend between the N outputs of the demultiplexer and the corresponding input waveguides to which the N outputs are coupled. The optical switches may be used to simultaneously switch all of a plurality of signals in a wavelength or frequency band to a different destination.
In another aspect of the invention the angular spacing of consecutively adjacent ones of the output waveguides is characterized by the magnitude of the value R given by:   R  =      "LeftBracketingBar"                  (                                            ⅆ              2                        ⁢                          α              q                                            ⅆ                          q              2                                      )                    (                              ⅆ                          α              q                                            ⅆ            q                          )              "RightBracketingBar"  
having a value of 0.1 or more for at least some sets of adjacent ones of the output waveguides.
Here,   (            ⅆ              α        q                    ⅆ      q        )
is the discrete derivative of xcex1q, and   (                    ⅆ        2            ⁢              α        q                    ⅆ              q        2              )
is the discrete second derivative of xcex1q.
Preferably, for each of at least two of the plurality of input wave guides there exists a distinct group of the output waveguides having associated passbands such that ranges of wavelengths of the passbands corresponding to the distinct groups are non-overlapping and ranges of values of q corresponding to the waveguides in each of the distinct groups are non-overlapping.
A still further aspect of the invention provides an optical apparatus. The optical apparatus comprises at least first and second input waveguides; an input free propagation region optically coupled to the first and second input waveguides; an output free propagation region; and an arrayed waveguide grating optically coupling the input and output free propagation regions. The optical apparatus has M output waveguides, where M is an integer and Mxe2x89xa74. Each of the output waveguides is optically coupled to the output free propagation region at an angular position xcex1i relative to an axis of the arrayed waveguide grating, where i is an index which increases with angular position and 0xe2x89xa6ixe2x89xa6Mxe2x88x921. No output waveguides are located between any two adjacent ones of the output waveguides. Each of a first Mxe2x88x921 of the output waveguides is separated from an adjacent one of the output waveguides by an angular spacing xcex94i=|xcex1i+1xe2x88x92xcex1i|. xcex94i varies periodically with i.
In preferred embodiments of the invention, when i has some value a, |xcex94axe2x88x92xcex94a+1| greater than 0.005xc3x97|xcex94a+xcex94a+1|. Most preferably, |xcex94axe2x88x92xcex94a+1 greater than 0.05xc3x97|xcex94a+xcex94a+1|.
Further features and advantages of the invention are discussed below.